The steady and dramatic decline in the sea ice cover of the Arctic Ocean over the last three decades has become a focus of media and public attention. At the opposite end of Earth, however, something more complex is happening. A new NASA study shows that from 1978 to 2010 the total extent of sea ice surrounding Antarctica in the Southern Ocean grew by roughly 6,600 square miles every year, an area larger than the state of Connecticut. And previous research by the same authors indicates that this rate of increase has recently accelerated, up from an average rate of almost 4,300 square miles per year from 1978 to 2006.

 

"There's been an overall increase in the sea ice cover in the Antarctic, which is the opposite of what is happening in the Arctic," said lead author Claire Parkinson, a climate scientist with NASA's Goddard Space Flight Center, Greenbelt, Md. "However, this growth rate is not nearly as large as the decrease in the Arctic."

 

Earth's poles have very different geographies. The Arctic Ocean is surrounded by North America, Greenland and Eurasia. These large landmasses trap most of the sea ice, which builds up and retreats with each yearly freeze-and-melt cycle. But a large fraction of the older, thicker Arctic sea ice has disappeared over the last three decades. The shrinking summer ice cover has exposed dark ocean water that absorbs sunlight and warms up, leading to more ice loss.

 

On the opposite side of the planet, Antarctica is a continent circled by open waters that let sea ice expand during the winter but also offer less shelter during the melt season. Most of the Southern Ocean's frozen cover grows and retreats every year, leading to little perennial¹ sea ice in Antarctica.

 

Using passive-microwave data from NASA's Nimbus 7 satellite and several Department of Defense² meteorological satellites, Parkinson and colleague Don Cavalieri showed that sea ice changes were not uniform around Antarctica. Most of the growth from 1978 to 2010 occurred in the Ross Sea, which gained a little under 5,300 square miles of sea ice per year, with more modest increases in the Weddell Sea and Indian Ocean. At the same time, the region of the Bellingshausen and Amundsen Seas lost an average of about 3,200 square miles of ice every year.

 

Parkinson and Cavalieri said that the mixed pattern of ice growth and ice loss around the Southern Ocean could be due to changes in atmospheric³ circulation. Recent research points at the depleted⁴ ozone⁵ layer over Antarctica as a possible culprit. Ozone absorbs solar energy, so a lower concentration of this molecule⁶ can lead to a cooling of the stratosphere (the layer between six and 30 miles above Earth's surface) over Antarctica. At the same time, the temperate⁷ latitudes⁸ have been warming, and the differential in temperatures has strengthened the circumpolar（极地附近的） winds flowing over the Ross Ice Shelf.

 

"Winds off the Ross Ice Shelf are getting stronger and stronger, and that causes the sea ice to be pushed off the coast, which generates areas of open water, polynyas（冰间湖）," said Josefino Comiso, a senior scientist at NASA Goddard. "The larger the coastal⁹ polynya, the more ice it produces, because in polynyas the water is in direct contact with the very cold winter atmosphere and rapidly freezes." As the wind keeps blowing, the ice expands further to the north.

 

This year's winter Antarctic sea ice maximum extent, reached two weeks after the Arctic Ocean's ice cap experienced an all-time summertime low, was a record high for the satellite era of 7.49 million square miles, about 193,000 square miles more than its average maximum extent for the last three decades.

 

The Antarctic minimum extents, which are reached in the midst of the Antarctic summer, in February, have also slightly increased to 1.33 million square miles in 2012, or around 251,000 square miles more than the average minimum extent since 1979.

 

The numbers for the southernmost ocean, however, pale in comparison with the rates at which the Arctic has been losing sea ice -- the extent of the ice cover of the Arctic Ocean in September 2012 was 1.32 million square miles below the average September extent from 1979 to 2000. The lost ice area is equivalent to roughly two Alaskas.

 

Parkinson said that the fact that some areas of the Southern Ocean are cooling and producing more sea ice does not disprove a warming climate.

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